This invention relates to a field effect transistor and, more particularly, to a Schottky barrier field effect transistor (SB FET) for high frequency application. This SB FET is also called "metal semiconductor field effect transistor (MES FET)".
A high frequency Schottky barrier FET is formed on a semiinsulator substrate allowing for a great mobility, for example, a substrate prepared from gallium arsenide (GaAs). A mobility of a GaAs substrate is several times greater than that of a silicon substrate. Already known is a GaAs SB FET having the following construction. This GaAs SB FET comprises source and drain regions which contain a high impurity concentration and formed in the surface area of the GaAs substrate and a channel region which contains a low impurity concentration and formed in a space defined between the source and drain regions. The GaAs SB FET is further provided with a gate electrode formed in Schottky contact with the channel region and also with source and drain electrodes formed in ohmic contact with the source and drain regions respectively. These source and drain electrodes are disposed on the corresponding source and drain regions at an interval equal to the length of the channel region. The source and drain electrodes respectively have a 2-ply structure constructed by mounting a platinum layer on a layer of gold-germanium alloy. The gate electrode is formed on a channel region between the source and drain electrodes.
The above-mentioned GaAs SB FET has the drawbacks that the product has noticeable variation in its various characteristics such as the gain in the high frequency band and noise factor, withstand voltage previously between the source and drain, and an extremely low yield. In other words, the GaAs SB FET constructed as described above has been found unadaptable for volume production because of low reproducibility.